An Internet protocol (IP) multimedia subsystem (IMS) is drawing attention as a core communication system for providing a new service in an IP-based next generation communication environment. The IMS provides a variety of multimedia services in a mobile communication network environment and integrates an IP network and a mobile communication environment as a set of nodes for performing call control. Therefore, the IMS facilitates an interoperation between a user terminal and the IP network, so that an IP-based multimedia service can be provided by using the user terminal. The IMS consists of three layers, that is, a connectivity layer for connection to the IMS, a control layer for managing call and session routing, accounting, and file protocols, and an application layer for storing and managing data and generating a service for a subscriber.
The IMS can support a variety of contents types (e.g., voice, multimedia data, text, etc.) by interoperating with an existing telephone network (i.e., public switched telephone network (PSTN)) and the Internet. An IP-based multimedia service provided by the IMS can be roughly classified into a messaging service, a voice service, and a video service. The message service includes a short messaging service (SMS), a multimedia messaging service (MMS), an e-mail service, a session initiation protocol (SIP) for instant messaging and presence leveraging extensions-instant message (SIMPLE-IM) service, an instant messaging and presence service (IMPS), etc. The voice service includes a voice over IP (VoIP) service, a push-to-talk over cellular (PoC) service, etc. The video service includes a video-over IP (Video-oIP), etc.
These services have their own protocol silos to transmit respective messages. That is, these services transmit the messages by using a transport protocol unique for each service. A transport protocol currently used by each service is summarized by Table 1 below.
TABLE 1Service TypeTransfer ProtocolSMSCall Signaling ProtocolMMSWSP (Wireless Session Protocol) or HTTP (Hyper textTransfer Protocol)E-mailIETF (Internet Engineering Taskforce Team) LemonadeIMAP (Internet Messaging Access Protocol),DS (Data Synchronization)SIMPLE-IMSIP (Session Initiation Protocol)/SIMPLE and MSRP(Message Session Relay Protocol)IMPSWSP or HTTPVoIPSIP/RTP (Real-time Transport Protocol)PoCSIP/RTPVideo_oIPSIP/RTP
Referring to Table 1, the IMS uses a session initiation protocol (SIP) which is a signaling protocol usable for the IP-based multimedia service. The SIP is a call control protocol of the application layer for generating, modifying, and ending an IP-based multimedia service session between user terminals or between a user terminal and an entity having an IP address by finding a location of a remote user terminal to which communication is desired.
In an IMS system, a plurality of user equipments (UEs) may be involved in a collaborative session. In this case, one UE can control media flows within the collaborative session, whereas the remaining UEs perform transmission and reception of the media flows. As such, a UE that provides service control for the collaborative session is referred to as a controller UE, and controlled UEs that provide the media flows to the collaborative session are referred to as controllee UEs. A controller UE may provide media flows for a collaborative session. A function of transferring a media flow for a specific UE to a different UE, adding a media flow to a different UE or removing a media flow generated in the different UE is referred to as inter-UE transfer (IUT) of the media flow.
When performing the IUT, whenever a state or capability of a different UE changes, it is notified to a UE which requests notification of the state or capability information of the different UE. Among different UEs, some of them may not be able to perform the IUT depending on situation. If the change in the state or capability of the UE that cannot perform the IUT depending on its situation is notified to the different UE, a signaling overhead occurs. In addition, a controller UE may attempt to perform the IUT for a UE that cannot perform the IUT, which can cause a signaling overhead when the IUT operation fails. Accordingly, there is a need for a method of effectively performing the IUT.